1. Field of the Invention
This invention relates to a modulated fibre Bragg grating strain gauge assembly suitable particularly, but not exclusively, for absolute gauging of strain.
2. Discussion of Prior Art
Conventional electrical strain gauges require a relatively large number of electrical lead outs which increases their weight and expense. Additionally such conventional electrical strain gauges are subject to electromagnetic noise which can give rise to false or distorted readings. This makes such conventional gauges relatively unsatisfactory for use on aircraft either as the retrofit assembly or as part of a composite panel assembly. Moreover in an aircraft application the use of electrical cables means that they can be damaged by lightening strikes with consequent reduction in reliability and life of the strain gauge assembly. Fibre optic strain gauges, such as is disclosed in patent application EP-840091-A2, offer particular advantages in airframe structures in terms of health monitoring as they are passive, can be embedded into composite structures, are not subject to lightening strikes and electromagnetic interference, and can be considered xe2x80x98fit for lifexe2x80x99 sensors. However, the fibre optic sensors presently used for strain gauge monitoring, namely the fibre Fabry perot interferometer and the fibre Bragg grating, each have practical limitations associated therewith. The fibre Fabry perot interferometer, although it achieves a high strain resolution and an excellent signal to noise ratio, is not an absolute gauge and it is difficult to multiplex a matrix of these devices over a fibre optic network. The fibre Bragg grating can achieve absolute gauging of strain and is better suited to multiplexing than the fibre Fabry perot interferometer, but the signal to noise ratio is poor both as a result of the conventional broadband light source used, and because standard fibre Bragg gratings only reflect light over a narrow waveband.
There is therefore a need for a fibre optic strain gauge sensor which has a good signal to noise ratio, a sufficiently wide reflection waveband to measure strain over a desired strain range, provides absolute gauging of strain, and allows multiplexing of many sensors from common light sources.
According to a first aspect of the present invention there is provided a modulated fibre Bragg grating strain gauge assembly for absolute gauging of strain including at least one sensor element in the form of a length of optical fibre containing, along part its length, means for partially reflecting light, means for generating and passing a beam of light with a spectral feature less than 0.1 nanometers in width into the at least one sensor element where reflection takes place, which reflection is a substantially sinusoidal intensity variation in wavelength over a range of from 2 to 3 nanometers comprising at least two substantially sinusoidal periods such that as the at least one sensor element sustains a change in length resulting from a strain thereon, the reflected intensity varies substantially sinusoidally along the at least two sinusoidal periods, means for receiving and processing the reflected light to establish the light intensity values at one (1f) and two times (2f) a modulation frequency (1f) applied to the means for generating the beam of light, and means to determine an absolute direction and magnitude of strain from a ratio of the intensity values 1f:2f.
Preferably the means for generating and passing a beam of light into the at least one sensor element includes a light generating source in the form of a semiconductor laser, a non-return isolator for receiving the beam of light from the laser, a waveguide splitter unit for splitting the light received from the isolator into at least one output light beam and at least one coupler, each for receiving one of the at least one output light beams and for passing it into one of the at least one sensor elements.
Conveniently each coupler is each operable to split the received light beam into two sub-beams, one of which is passed directly into one of the at least one sensor elements via a single mode optical fibre, and to receive from the respective at least one sensor element the light reflected therefrom.
Advantageously the means for receiving and processing the reflected light includes at least one photodetector for receiving the reflected light from the coupler, at least one pair of lockin detectors operable to sample the reflected light output from the photodetector, one at a frequency of 1f and the other at a frequency of 2f, and demodulate the samples at these two frequencies, and means for logging the demodulated samples and for establishing the phase relationship between the 1f and 2f values.
Preferably the means for partially reflecting light is a single Bragg grating.
Conveniently the laser source light is tuneable to a midpoint of the single Bragg grating.
Advantageously the means for partially reflecting light is a series of single Bragg gratings, each of which gratings resonates at a unique frequency.
According to a further aspect of the present invention the means for generating and passing the beam of light into the at least one sensor element includes a plurality of light generating sources each in the form of a semiconductor laser, each of which lasers is uniquely tuned to a midpoint of one of the series of single Bragg gratings and has a unique and selectable frequency modulation 1fi imposed thereon, a plurality of non-return isolators for receiving an output from each of the corresponding lasers, a combining unit for combining the outputs from each of the plurality of isolators, which combined outputs provide at least one output light beam passable into each of the at least one sensor elements, and at least one coupler each for receiving one output light beam from the combining unit and passing it into one of the at least one sensor elements.
Preferably the means for receiving and processing the reflected light from each of the series of single Bragg gratings includes at least one photodetector for receiving the reflected light from the respective coupler and a plurality of pairs of lockin detectors operable to sample the reflected light output from the detector, one at each corresponding 1fi and the other at each corresponding 2fi frequency.
Conveniently the or each single Bragg grating is of a type made by a programmable ultra violet interference fabrication process operable to impose a variation in refractive index depth and pitch therealong, thereby providing the required substantially sinusoidal variation of reflected light intensity.
Advantageously the ultra violet interference system is operable to impose a linearly varying pitch on the or each single Bragg grating.
Preferably the substantially sinusoidal variation of reflected light intensity has two periods, which two periods are provided by the ultra violet interference system operable to impose a sinusoidal refractive index depth variation along the or each single Bragg grating, which sinusoidal refractive index depth variation has a frequency and amplitude selectable to provide no more than four periods therealong.
Conveniently the substantially sinusoidal variation of reflected light intensity has three periods, which three periods are provided by the ultra violet interference system operable to impose a sinusoidal refractive index depth variation along the or each single Bragg grating, which sinusoidal refractive index depth variation has a frequency and amplitude selectable to provide at least five periods therealong.
Advantageously the substantially sinusoidal variation of reflected light intensity has three or more periods, which three or more periods are provided by the ultra violet interference system operable to superimpose a sinusoidal variation of pitch on the linearly varying pitch.
Preferably the means to determine an absolute direction and magnitude of strain from the strain gauge assembly includes calibrating means for calibrating the ratio of intensity values 1f:2f, and means for locating the ratio of 1f:2f within one of the at least two substantially sinusoidal periods.
Conveniently the calibrating means includes a first calibration Bragg grating, a second calibration Bragg grating, the waveguide splitter unit having three output light beams, three couplers and three sensor elements.
Advantageously the first calibration Bragg grating is locatable along part of a first of the three output sensor elements, is unstrained and locatable in a controlled temperature environment thereby providing a temperature stabilised ratio of the intensity values (1f:2f)T, the single Bragg grating is locatable along a second of the three output sensor elements, and the second calibration Bragg grating is locatable along a third of the three output sensor elements, is unstrained, subject to substantially identical environmental conditions to the single Bragg grating and provides an unstrained ratio of the intensity values (1f:2f)U, which unstrained ratio (1f:2f)U, temperature stabilised ratio (1f:2f)T and the means for locating the ratio 1f:2f within one of the at least two sinusoidal periods are combinable with the ratio of the intensity values 1f:2f relating to the single Bragg grating, to determine the phase relationship from which an absolute direction and magnitude of the strain on the single Bragg grating can be established.
Preferably the calibrating means includes a series of first calibration Bragg gratings, each of which first calibration Bragg gratings is locatable along one of the at least one sensor elements, is unstrained and locatable in a controlled temperature environment thereby providing a series of temperature stabilised ratios of the intensity values (1fi:2fi)T, and a series of second calibration Bragg gratings, each of which second calibration gratings is locatable along one of the at least one sensor elements, is unstrained, subject to substantially identical environmental conditions to each respective single Bragg grating and provides a series of unstrained ratios of the intensity values (1fi:2fi)U, which unstrained ratios (1fi:2fi)U, temperature stabilised ratios (1fi:2fi)T and the means for locating the ratios 1fi:2fi within one of the at least two sinusoidal periods are combinable with the ratio of the intensity values 1fi:2fi relating to each respective single Bragg grating, to determine the phase relationship from which an absolute direction and magnitude of the strain on each single respective Bragg grating can be established.
Conveniently each of said first and second calibration Bragg gratings is of a type made by a programmable ultra violet interference fabrication process operable to impose a variation in refractive index depth and pitch therealong, thereby providing a substantially sinusoidal variation of reflected light intensity.
Advantageously the series of single Bragg gratings and the series of first and second calibration gratings each includes five single Bragg gratings.
Preferably there are provided five sensor elements and five photodetectors.